Normalization method for a chronically implanted optical sensor

ABSTRACT

A system and method are provided for estimating blood oxygen saturation independent of optical sensor encapsulation due to placement in blood, where the blood includes a blood flow characteristic of: a relatively low, a stasis, a stagnant value. The method includes determining tissue overgrowth correction factor that includes optical properties of the tissue that cause scattering of the emitted light to a detector and relative amplitudes of the emitted light wavelengths. A corrected time interval measured for infrared light is based on an infrared signal and a corrected time interval for red light is determined by subtracting red light signal due to presence of tissue overgrowth. The red light signal due to tissue overgrowth is proportional to total infrared signal less nominal infrared signal. Oxygen saturation is estimated based on standard calibration factors and the ratio of the corrected infrared time interval and the corrected red time interval.

FIELD OF THE INVENTION

The present invention relates generally to the field of implantableoptical sensors and more specifically to a method for providing accurateoptical sensing of blood oxygen saturation in the presence of tissueovergrowth on the optical sensor.

BACKGROUND OF THE INVENTION

Implantable medical devices (IMDs) for monitoring a physiologicalcondition or delivering a therapy typically rely on one or more sensorspositioned in a patient's blood vessel, heart chamber, or other portionof the body. Examples of IMDS include heart monitors, pacemakers,implantable cardioverter-defibrillators (ICDs), myostimulators, nervestimulators, drug delivery devices, and other IMDs where such sensorsare desirable. Implantable sensors used in conjunction with an IMDgenerally provide a signal related to a physiological condition fromwhich a patient condition or the need for a therapy can be assessed.

Measurement of blood oxygen saturation levels are of interest indetermining the metabolic state of the patient. Generally, a decrease inblood oxygen saturation is associated with an increase in physicalactivity or may reflect insufficient cardiac output or respiratoryactivity. Thus monitoring blood oxygen saturation allows an implantablemedical device to respond to a decrease in oxygen saturation, forexample by pacing the heart at a higher rate. An implantable oxygensensor for use with an implantable medical device is generally disclosedin commonly assigned U.S. Pat. No. 6,198,952 issued to Miesel, herebyincorporated herein by reference in its entirety. Cardiac pacemakersthat respond to changes in blood oxygen saturation as measured by

CROSS REFERENCE TO RELATED APPLICATIONS

This present patent disclosure is a continuation of that certainco-pending application of common title which issued to Jonathan Robertson 13 Sep. 2005 as U.S. Pat. No. 6,944,488, the contents of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to the field of implantableoptical sensors and more specifically to a method for providing accurateoptical sensing of blood oxygen saturation in the presence of tissueovergrowth on the optical sensor; in particular the invention providessignificant utility when directed toward volume of blood and said bloodhas a characteristic flow-rate which in exemplary embodiments includesone of a relatively low rate, a stasis rate, a stagnant rate.

BACKGROUND OF THE INVENTION

Implantable medical devices (IMDs) for monitoring a physiologicalcondition or delivering a therapy typically rely on one or more sensorspositioned in a patient's blood vessel, heart chamber, or other portionof the body. Examples of IMDS include heart monitors, pacemakers,implantable cardioverter-defibrillators (ICDs), myostimulators, nervestimulators, drug delivery devices, and other IMDs where such sensorsare desirable. Implantable sensors used in conjunction with an IMDgenerally provide a signal related to a physiological condition fromwhich a patient condition or the need for a therapy can be assessed.

Measurement of blood oxygen saturation levels are of interest indetermining the metabolic state of the patient. Generally, a decrease inblood oxygen saturation is associated with an increase in physicalactivity or may reflect insufficient cardiac output or respiratoryactivity. Thus monitoring blood oxygen saturation allows an implantablemedical device to respond to a decrease in oxygen saturation, forexample by pacing the heart at a higher rate. An implantable oxygensensor for use with an implantable medical device is generally disclosedin commonly assigned U.S. Pat. No. 6,198,952 issued to Miesel, herebyincorporated herein by reference in its entirety. Cardiac pacemakersthat respond to changes in blood oxygen saturation as measured by anoptical sensor are generally disclosed in U.S. Pat. No. 4,202,339 issuedto Wirtzfeld and in U.S. Pat. No. 4,467,807 issued to Bornzin.

One limitation encountered with the use of implantable optical sensorscan arise as the result of tissue encapsulation of the sensor thatoccurs as a result of the body's normal response to a foreign object. Ifan optical blood oxygen sensor is positioned in an area of relativelyhigh blood flow, tissue encapsulation of the sensor may not occur or mayat least be minimized to a thin collagenous sheath. If a blood oxygensensor resides in an area of relatively low blood flow or a stagnantarea, tissue encapsulation is likely to occur and the capsule may becomerelatively thick. Such tissue overgrowth interferes with the performanceof the sensor in accurately measuring blood oxygen or other metabolitesby reducing the (light) signal to noise ratio. For example, the lightsignal associated with blood oxygen saturation is reduced due toattenuation of emitted light from the optical that reaches the bloodvolume and attenuation of the reflected light from the blood volumereaching a light detector included in the optical sensor. Noise due toextraneous light reaching the light detector is increased by thescattering of emitted light by the tissue overgrowth.

The time course and degree of tissue encapsulation of an optical sensor,or any other medical device implanted within the blood volume, isuncertain. Thrombus formation in the vicinity of the sensor due to bloodstasis or endothelial injury can occur at unpredictable times afterdevice implant. If the thrombus is in contact with the endocardium orendothelium, macrophages can invade the clot, phagocytose the bloodcells and orchestrate collagenous encapsulation by fibroblasts. Becausethe time course and occurrence of these events is unpredictable, thereliability of blood oxygen saturation measurements at any point in timemay be uncertain.

One approach to solving the problem of tissue overgrowth is generallydisclosed in U.S. Pat. No. 6,125,290 issued to Miesel, incorporatedherein by reference in its entirety. A self-test light detector isprovided for estimating the amount of light reflected back into a lightemitter portion instead of being transmitted through a lens forreflection from a blood volume. An output signal from self-test lightdetector may be employed to calibrate or adjust the output signalprovided by a light detector in a manner that the estimate of bloodoxygen saturation is compensated or adjusted to account for the degreeor amount of tissue overgrowth of the sensor.

A need remains, however, for a method for adjusting a blood oxygensaturation (or other metabolite) measurement to account for extra lightintensity associated with light scattering tissue or thrombus over theoxygen sensor, and the like. The method preferably provides accurateblood oxygen saturation measurement independent of the presence oftissue overgrowth.

SUMMARY OF THE INVENTION

In an exemplary embodiment, an implantable optical sensor system andmethod are provided for accurately estimating blood oxygen saturationindependent of the presence of tissue encapsulation of the opticalsensor. According to the present invention, dual wavelength radiation isused to beneficially provide a first wavelength signal substantiallyindependent of the presence of a metabolite and a second wavelengthsignal is substantially dependent upon the presence of said metabolite.

Thus, in the exemplary embodiment, a two wavelength optical sensor isemployed wherein the amount of reflected light from one wavelength,typically red, is dependent on oxygen saturation, and the secondwavelength, typically infrared light, is independent of oxygensaturation. A time interval is measured for each light wavelength as thecurrent induced on a sensor (e.g., a photodetector) in response to theintensity of the reflected light integrated over a capacitor included inthe photodetector. The time interval measured for the red light signalis normalized by the time interval measured for the infrared lightsignal to account for differences in hematocrit and blood flow velocity.

The method includes calculating a corrected time interval measured forthe red light signal and a corrected time interval measured for theinfrared light signal to account for the presence of tissue overgrowth.A tissue overgrowth correction factor is used in calculating a correctedred light time interval which accounts for: 1) the optical properties ofthe tissue that cause scattering of the emitted light to a lightdetector determined experimentally, and 2) the relative amplitudes ofthe emitted light wavelengths from the optical sensor (e.g., asdetermined at the time of device manufacture). A corrected time intervalmeasured for infrared light is determined based on the nominal infraredsignal returned from an inspected volume of blood in the absence oftissue overgrowth (e.g., as determined at the time of devicemanufacture). A corrected time interval measured for red light isdetermined by subtracting the amount of red light signal attributed tothe presence of tissue overgrowth from the total red light signal. Theamount of red light signal attributed to the presence of tissueovergrowth is calculated by multiplying the total infrared signal (lessthe nominal infrared signal) by the tissue correction factor. Oxygensaturation is estimated based on standard calibration factors and theratio of the corrected infrared time interval and the corrected red timeinterval.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged view of an exemplary oxygen sensor assembly thatmay be included in an implantable medical lead and with which thepresent invention may be usefully practiced.

FIG. 2 is a sectional view of the oxygen sensor assembly of FIG. 1.

FIG. 3 shows a block diagram of an implantable medical device systemincluding an implantable medical device (IMD) and medical electricallead 8 having an optical sensor assembly for use in sensing blood oxygensaturation.

FIG. 4 is a flow chart summarizing steps included in a method foraccurately estimating blood oxygen saturation using an implantableoptical sensor independent of tissue overgrowth on the sensor.

FIG. 4B is a flow chart summarizing steps included in an alternativemethod for accurately estimating blood oxygen saturation using animplantable optical sensor independent of tissue overgrowth on thesensor.

FIG. 5 is a graph of oxygen saturation measurements made from achronically implanted oxygen sensor having tissue overgrowth based onprior art methods and based on the tissue overgrowth correction methodprovided by the present invention plotted in comparison to a referenceoxygen saturation measurement.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed at providing a method for accuratelymeasuring changes in blood oxygen saturation using a chronicallyimplantable optical sensor. In particular, the present inventionprovides a method for compensating for extra light intensity detected bythe optical sensor due to light scattering matter, typically collagenoustissue or thrombus, which may be present over the sensor. The methodsincluded in the present invention employ a two-wavelength system and acorrection method that allows the extra light intensity due to lightscattering tissue over the sensor to be compensated for in calculatingoxygen saturation.

Two-wavelength optical sensing systems are known for use in the field ofblood oximetry. See for example, U.S. Pat. No. 3,847,483 issued to Shawet al. In a two-wavelength system, the reflected light signal of onewavelength that changes in intensity with blood oxygen saturation isnormalized by a second wavelength of which the reflected intensity isindependent of blood oxygen saturation but dependent on otherphysiological changes in the measured blood volume, such as blood flowvelocity and hematocrit concentration. The methods included in thepresent invention provide an additional correction method to account foradditional light signal detected due to tissue overgrowth. The intensityof both wavelengths in a two-wavelength system will be changed in thepresence of light scattering matter over the sensor. The presence oflight scattering matter over the sensor is expected to affect theintensity of both wavelengths used in sensing oxygen. Hence, as willcome to be understood in the description herein, the extra intensity ofdetected light due to the presence of light scattering matter proximatethe sensor can be determined based on the intensity of the wavelengththat is not affected by changes in blood oxygen saturation.

FIG. 1 is an enlarged view of an exemplary oxygen sensor assembly 10that may be included in a medical lead and with which the presentinvention may be usefully practiced. An elongated sensor housing 12 isprovided for housing oxygen sensor components. Lenses 14 and 16 areprovided for passing emitted and reflected light from a light emitterand to a light detector, respectively, both of which reside withinhousing 12. Although a single lens may be used in lieu of two lenses.Lead body 7 is provided for carrying insulated conductors from thecircuitry included in sensor assembly 10 to a connector assembly (notshown) located at a proximal end of the medical lead used formechanically and electrically connecting the lead to an implantablemedical device. A lead carrying oxygen sensor assembly 10 mayadditionally include other types of sensors and/or electrodes accordingto the intended use of the lead.

FIG. 2 is a sectional view of oxygen sensor assembly 10 of FIG. 1. Lightemitters 20 and 22 are mounted on an oxygen sensor hybrid 30 in a lightemitting portion 19 of oxygen sensor 10. Each light emitter 20 and 22emits a different wavelength. Typically one of emitters 20 and 22 emitsred light and the other of emitters 20 and 22 emits infrared light.Emitted light passes through lens 14 and must also pass through anytissue overgrowth 40 that is present on all or a portion of lens 14before it enters blood volume 50. An optional self-test light detector32 is shown in the light emitter portion 19 of oxygen sensor assembly10. In one embodiment of the present invention, self-test light detector32 is included for detecting the presence of tissue overgrowth asdisclosed in the above-cited '290 patent issued to Miesel.

Light that is reflected from the blood volume 50 must pass through anytissue overgrowth 40 present over all or a portion of lens 16 before itreaches light detector 18. Light detector 18 is mounted on oxygen sensorhybrid 30 in a light detector portion 17 of oxygen sensor 10. Timeintervals that are inversely proportional to the intensity of thereceived light signals associated with each of the two measuredwavelengths, red and infrared, are measured as an electrical currentinduced in light detector 18 (which is preferably integrated over acapacitor included in the light detector 18). The presence of tissueovergrowth 40 generally reduces the light signal to noise ratio, asdescribed previously, by both attenuating the reflected light signalassociated with blood oxygen saturation and increasing noise due toextraneous light scattered by tissue overgrowth 40 reaching detector 18.The methods of the present invention address the latter problem ofincreased extraneous light received by detector 18, resulting inshortened time intervals, by correcting the measured time intervals forthe amount of light signal attributed to noise.

FIG. 3 shows a block diagram of an implantable medical device systemincluding an implantable medical device (IMD) 11 and medical electricallead 8 having an optical sensor assembly 10, for use in sensing bloodoxygen saturation. Optical sensor 10 is typically mechanically andelectrically coupled to the distal ends of lead conductors disposedwithin the body 7 of lead 8. Connector elements disposed at the proximalend of lead 8 are connected to the proximal ends of lead conductors andprovide connection to sensor drive circuit 80 and sensor processorcircuit 82 via a connector block 86. Sensor drive circuit 80 providesthe operational power for optical sensor 10 and controls the timing ofoptical sensor operation. Sensor process circuit 82 receives opticalsensor signal output and processes the signal output to estimate ameasurement of blood oxygen saturation. In accordance with the presentinvention, the methods used by sensor process circuit 82 for estimatingblood oxygen saturation includes calculations made to correct for thepresence of tissue overgrowth covering a portion of or all of the lenses14, 16 of optical sensor 10.

When IMD 11 includes cardiac pacing capabilities, lead 8 mayadditionally include pacing, sensing and/or defibrillation electrodesgenerally disposed at the distal end of lead 8 in operative relation toone or more heart chambers. Alternatively, additional pacing and sensingleads are included in the IMD system. Cardiac pacing and sensing controlcircuitry, a clock, and a battery for powering IMD operations areincluded in IMD circuits 84. Detailed descriptions of such circuitryincluded in an implantable medical device and its operation are providedin the above-incorporated '952 patent to Miesel.

Preferably, the reflection of red light (wavelength ˜660 nm) and thereflection of infrared light (wavelength ˜880 nm) are used for measuringblood oxygen saturation. The absorption of red light is a function ofoxygen saturation and therefore the intensity of the reflected red lightsignal received by a light detector is inversely proportional to oxygensaturation. The absorption of infrared light is a function ofnon-oxygenated hemoglobin and hematocrit concentration. The ratio of thered light intensity to the infrared light intensity of the reflectedlight is used to normalize a measured light signal for changes inhematocrit and blood flow velocity. Prior art oxygen sensors thereforedetermine the blood oxygen saturation (SAO₂) according to the inverserelation to the reflected red light intensity normalized by thereflected infrared light intensity as shown by Equation (1):(1) SAO₂ =A+B(1/(R/IR))

-   -   (2) wherein A and B are calibration constants defined by the        intercept and slope of a calibration curve, respectively; IR is        the intensity of reflected infrared light, and R is the        intensity of reflected red light.

An alternative and commonly used form of Equation (1) takes thelogarithm of the term including the ratio of the red to infrared lightsignal in order to linearize the function as shown in Equation (1′):(1) SAO₂ =A+B log(1/(R/IR)).  (1′)

For the sake of simplicity, the methods described herein will beelaborated on with regard to Equation (1), but substitutions to bedescribed in detail below to be made in Equation (1) for correcting forthe presence of tissue overgrowth may be equivalently made in Equation(1′).

-   -   (1) Equation (1) may be re-written as:        SAO₂ =A+B(IR/R).  i.

In accordance with the present invention, the terms R and IR inEquations (1) and (2) are corrected to account for increased lightsignal due to tissue overgrowth by subtracting the amount of red lightand infrared light signal attributed to tissue overgrowth.

In correcting for the presence of tissue overgrowth, an assumption ismade that the optical properties of the tissue overgrowth will affectboth red and infrared light wavelengths and that the amount of red lightreflected by tissue overgrowth will be proportional to the amount ofinfrared light reflected by tissue overgrowth. Another assumption madeis that tissue overgrowth on the optical sensor has the same effect onsensor measurements as a shortened “dark interval.” Herein, the term“dark interval” refers to the time interval measured from aphotodetector in the absence of any true light signal reflected from theblood. The dark interval therefore is the time interval measured due tostray current or extraneous light reaching the photodetector such aslight leakage from the emitter to the detector portion of the oxygensensor. Hence, the presence of tissue overgrowth will increaseextraneous light reaching the photodetector, causing a shortened darkinterval. Another underlying assumption is that enough light is passedthrough the tissue to be reflected by the passing blood volume.

Based on the above assumptions, the “dark interval” measured in thepresence of tissue overgrowth will include an infrared signal componentand a red signal component. The infrared time interval contributing tothe dark interval, T_(IRdark), is given by Equation (3):(1) T _(IRdark)=1/(IR−IR _(nominal))  (3)

-   -   (2) wherein IR is the total infrared light signal received from        all sources and IR_(nominal) is the expected infrared light        signal to be reflected from a blood pool when no tissue        overgrowth is present.

The terms representing the IR and IR_(nominal) light signals in Equation3 can be replaced by the inverse of the time interval measured for eachlight signal. Hence, Equation (3) can be rewritten as:(1) T _(IRdark)=1/((1/T _(IR))−(1/T _(IRnom))  (4)

-   -   (2) wherein T_(IR) is the time interval measured from the        photodetector due to the total infrared light signal and        T_(IRnom) is the time interval measured from the photodetector        due to the expected infrared light signal reflected from a blood        pool in the absence of any tissue overgrowth.

As stated above, an assumption is made that the optical properties ofthe tissue cause red light to be reflected proportionally to infraredlight. The time interval contribution to the dark interval from redlight reflected from the tissue overgrowth, TR_(dark), can therefore beexpressed as:(1) T _(Rdark) =K(T _(IRdark)),  (5)

-   -   (2) wherein K is a correction factor that takes into account: 1)        the ratio of red to infrared light intensities reflected from        tissue overgrowth when red and infrared light sources emit equal        intensities of red and infrared light, and 2) a scaling factor        that represents the relative amplitudes of red and infrared        light output power from the light emitters of the oxygen sensor.        T_(IRdark) is given by Equation (4) above.

A corrected red time interval, T_(Rcorrected), for estimating oxygensaturation can be obtained from the inverse of the total red lightsignal received, R, less the red light signal attributed to the tissueovergrowth, R_(dark):(1) T _(Rcorrected)=1/(R−R _(dark)).  (6)

-   -   (2) The terms R and R_(dark) in Equation 6 may be substituted        for by the inverse of the corresponding time intervals measured        by the photodetector:        (3) T _(Rcorrected)=1/((1/T _(R))−(1/T _(Rdark))),  (7)    -   (4) wherein T_(R) is the time interval measured due to the total        amount of reflected red light at the photodetector and T_(Rdark)        is given by Equation (5) above.

The corrected infrared time interval, T_(IRcorrected), is simply equalto the time interval measured for infrared light reflected from a volumeof blood in the absence of any tissue overgrowth:(1) T _(IRcorrected) =T _(IRnom)  (8)

-   -   (2) Thus, in accordance with the present invention, Equation (2)        above can be rewritten as:        (3) SAO₂ =A+B(IR _(corrected) /R _(corrected)),  (9)    -   (4) which can be rewritten in terms of measured time intervals        as:        (5) SAO₂ =A+B(T _(Rcorrected) /T _(IRcorrected))  (10)

By using Equations 4, 5, 7, and 8, to substitute for T_(Rcorrected) andT_(Ircorrected), and after mathematical simplification and reduction,Equation 10 can be equivalently rewritten as Equation 11 below in termsthat are either measured or known quantities:(1) SAO₂ =A+B{(K*T _(R) *T _(IRnom))/((K*T _(IR) *T _(IRnom))−(T _(R)(T_(IRnom) −T _(IR))))}.  (11)

As indicated previously, constants A and B are standard calibrationconstants; K is determined by experimentally measuring the ratio of redto infrared light reflected by tissue overgrowth upon exposure to equalintensities of emitted red and infrared light for a given sensorgeometry and by knowing the relative output of the red and infraredlight emitters of the oxygen sensor at the time of manufacture;T_(IRnominal) is determined experimentally by measuring the infraredtime interval due to infrared light reflected from a volume of blood inthe absence of any tissue overgrowth; and T_(R) and T_(IR) are the timeintervals measured from the photodetector associated with the intensityof red and infrared light reflected into the detector portion of theoxygen sensor during oxygen sensing operations.

FIG. 4A is a flow chart summarizing steps included in a method foraccurately estimating blood oxygen saturation using an implantableoptical sensor independent of tissue overgrowth on the sensor. Method100 introduces the use of two new constants, K and T_(IRnom), as definedabove, in an equation for calculating oxygen saturation from opticalsensor measurements. At step 105, the value for the tissue overgrowthcorrection factor, K, is obtained by experimentally quantifying theratio of equally applied intensities of red light to infrared lightreflected from tissue overgrowth for a given sensor geometry andmultiplying this ratio by a scaling factor that represents the relativeamplitudes of red and infrared light output power from the lightemitters of the oxygen sensor as determined at the time of manufactureof the sensor. The ratio of reflected red to infrared light representsthe light scattering and reflecting properties of tissue encapsulating achronically implanted oxygen sensor. The scaling factor is known at thetime of sensor manufacture.

At step 107, T_(IRnom) is determined. T_(IRnom) is also determined atthe time of sensor manufacture and can be determined by measuring thereflected infrared light from a sample of material with known scatteringand reflecting properties of IR light. T_(IRnom) represents the returninfrared light from a volume of blood at any known level of oxygensaturation.

At step 110 the calibration constants A and B are determined as theslope and offset of an oxygen saturation curve. These constants are alsodetermined at the time of device manufacture, based upon experimentaldata to determine the response of a given sensor geometry, and the redand infrared output power of the emitters for an individual sensor. Thedetermination and use of these calibration constants is known in theprior art.

Steps 105 through 110 are performed experimentally or at the time ofsensor manufacture such that the constants, K, T_(IRnom), A and B, maybe programmed into firmware or software used by an implanted medicaldevice in calculating oxygen saturation using signals received from theoptical sensor. At step 112, the optical sensor is deployed with anassociated IMD system by implanting the sensor at a desired location inthe blood volume of a patient. Step 115 represents the normal operationof the optical sensor in which the intensities of red and infraredreflected light received at the light detector portion of the oxygensensor are measured. At step 120, oxygen saturation is calculatedaccording to Equation 11 above, based on the tissue overgrowthcorrection methodology provided by the present invention.

FIG. 4B is a flow chart summarizing steps included in an alternativemethod for accurately estimating blood oxygen saturation using animplantable optical sensor independent of tissue overgrowth on thesensor. Identically labeled steps included in method 150 correspond tothe same steps included in method 100, described above. However, method150 includes a decision step 117 for determining if tissue overgrowth ispresent. Tissue overgrowth may be detected by the use of a self-testlight detector in the emitter portion of the oxygen sensor, as generallydisclosed in the above-cited '290 patent to Miesel. When tissueovergrowth is detected, Equation 11 is used by sensor processingcircuitry of the implantable medical device for calculating oxygensaturation corrected for the presence of tissue overgrowth at step 120.If tissue overgrowth is not detected at decision step 117, Equation 2may be used by sensor processing circuitry for calculating oxygensaturation without correcting for the presence of tissue overgrowth atstep 122.

FIG. 5 is a graph of experimental oxygen saturation measurements madefrom a chronically implanted oxygen sensor having tissue overgrowthbased on prior art methods and based on the tissue overgrowth correctionmethod provided by the present invention plotted compared to a referenceoxygen saturation measurement. The graph is exemplary of the operationof an oxygen sensor with and without the tissue overgrowth correctionmethod. The graph shows the results of oxygen saturation measurementsobtained from a chronically implanted oxygen sensor and an acutelyimplanted reference oxygen saturation sensor during an oxygendesaturation experiment. Oxygen desaturation was accomplished in asedated canine by temporarily introducing helium into the respirator,causing reduction in the uptake of oxygen.

Reference oxygen sensor measurements (square symbols) show a decrease inoxygen saturation followed by a return to normal oxygen saturationlevels after respirator oxygen levels had been restored. The chronicallyimplanted oxygen sensor in this canine has been overgrown with fibrin,causing a lack of response in the oxygen values determined withoutcorrecting for tissue overgrowth (triangle symbol) compared to thereference oxygen sensor measurements.

The tissue overgrowth correction in this example was determined usingred and infrared time intervals recorded at the time of implant, ratherthan at the time of manufacture, since tissue response had not beendetermined for the sensor before manufacture. The correction wasperformed using time intervals extracted from the oxygen saturation andthe measured infrared time intervals, since the red time intervals werenot recorded as part of the original study. This causes a decrease inresolution in the corrected oxygen saturation measurement that would notbe present in a system that either compensates the signals beforerecording, or with a system that records the red time interval alongwith the IR time interval as described above.

However, it can be seen from the graph of FIG. 5 that the response ofthe corrected oxygen saturation measurements made from the chronicallyimplanted, tissue overgrown sensor (circle symbols), responds to thedesaturation event similarly to the reference oxygen sensormeasurements, while the uncorrected oxygen saturation measurements showan attenuated response to the desaturation event.

Thus, a system and method are provided for accurately estimating bloodoxygen saturation from optical sensor measurements independent of thepresence of encapsulating tissue over all or a portion of the opticalsensor. Methods included in the present invention have been describedwith regard to an optical oxygen sensor application. However, it iscontemplated that tissue overgrowth correction methodologies provided bythe present invention may be applied in the use of other types ofimplantable optical sensors employing a two-wavelength system, such asglucose sensors. Depending on the wavelengths employed, the tissueovergrowth correction factor, K, may not be a constant if the opticalproperties of the tissue overgrowth affect the normalizing wavelengthdifferently than the targeted measurement wavelength. Therefore, whilethe present invention has been described according to specificembodiments presented herein, aspects of the present invention may beapplied in alternative embodiments including implantable, two-wavelengthoptical sensor systems. As such the disclosed embodiments are intendedto be exemplary, not limiting, with regard to the following claims.

1. A method for accurately estimating an oxygen saturation metric for avolume of blood by an optical sensor measurement, comprising: deployinga dual wavelength optical sensor into fluid communication with a volumeof blood, wherein said volume of blood is characterized by a blood flowrate condition, said flow rate condition comprising one of: relativelylow, stasis, substantially stagnant; determining a tissue overgrowthcorrection factor (K) for a first and a second wavelength of opticalradiation in a dual wavelength optical sensor, wherein said firstwavelength of optical radiation is substantially proportional to anamount of saturated oxygen present in said volume of blood and saidsecond wavelength of radiation is substantially independent to theamount of saturated oxygen present in the volume of blood; determining anominal time interval for detecting the second wavelength of opticalradiation (T2) after it is directed to a volume of blood having knownoptical properties; determining a pair of calibration constants (A,B)for said first and said second wavelength of optical radiation;measuring a first time interval for the first wavelength of radiationand a second time interval for the second wavelength of radiation whensaid first and second wavelengths of radiation are directed to thevolume of blood; and calculating a saturation metric for the saturatedoxygen based upon the tissue overgrowth correction factor (K), the pairof calibration constants (A,B), the nominal time interval, the firsttime interval and the second time interval.
 2. A method according toclaim 1, wherein the calculating step further comprises: calculatingoxygen saturation according to the following mathematical expression:SAO₂ =A+B{(K*T _(R) *T _(IRnom))/((K*T _(IR) *T _(IRnom))−(T _(R)(T_(IRnom) −T _(IR))))}.
 3. A method according to claim 1, furthercomprising providing an oxygen saturation metric output signal.
 4. Amethod according to claim 1, wherein after measuring the first timeinterval and the second time interval performing the step of: detectinga tissue overgrowth condition, and if the tissue overgrowth condition ispositive then: calculating a corrected oxygen saturation metric; and ifthe tissue overgrowth condition is negative then: calculating anuncorrected oxygen saturation metric.
 5. A method according to claim 1,wherein the first wavelength of optical radiation is approximately 660nm and the second wavelength of radiation is approximately 880 nm.
 6. Amethod according to claim 1, wherein the dual wavelength optical sensoris disposed in a volume of blood proximate an implantable medicaldevice.
 7. A method according to claim 6, wherein the implantablemedical device comprises: an implantable heart monitor, a pacemaker, acardioverter-defibrillator, a myostimulation device, a nerve stimulationdevice, a drug delivery device.
 8. A method according to claim 1,wherein said dual wavelength optical sensor mechanically couples to animplantable medical device.
 9. A method according to claim 1, whereinthe oxygen saturation metric comprises a saturated arterial oxygenmetric.
 10. A system for accurately estimating an oxygen saturationmetric for a volume of blood, comprising: means for determining a tissueovergrowth correction factor (K) for a first and a second wavelength ofoptical radiation in a dual wavelength optical sensor, wherein saidfirst wavelength of optical radiation is substantially proportional tosaturated oxygen present in a volume of blood and said second wavelengthof radiation is substantially independent of saturated oxygen; means fordetermining a nominal time interval for detecting the second wavelengthof optical radiation (T2) after it is directed to the volume of bloodhaving known optical properties; means for determining a pair ofcalibration constants (A,B) for said first and said second wavelength ofoptical radiation; means for placing the dual wavelength optical sensorin the volume of blood, wherein said volume of blood is characterized bya blood flow-rate condition, said flow-rate condition comprising one of:relatively low, stasis, substantially stagnant; means for measuring afirst time interval for the first wavelength of radiation and a secondtime interval for the second wavelength of radiation when said first andsecond wavelengths of radiation are directed to the volume of blood; andmeans for calculating a saturation metric for the metabolite of interestbased upon the tissue overgrowth correction factor (K), the pair ofcalibration constants (A,B), the nominal time interval, the first timeinterval and the second time interval.
 11. A system according to claim9, further comprising means for calculating oxygen saturation for thevolume of blood according to the following mathematical expression:SAO₂ =A+B{(K*T _(R) *T _(IRnom))/((K*T _(IR) *T _(IRnom))−(T _(R)(T_(IRnom) −T _(IR))))}.
 12. A system according to claim 10, furthercomprising providing an oxygen saturation-metric output signal.
 13. Asystem according to claim 10, further comprising: means for detecting atissue overgrowth condition, and wherein if the tissue overgrowthcondition is positive then calculating a corrected oxygen saturationmetric; and wherein if the tissue overgrowth condition is negative thencalculating an uncorrected oxygen saturation metric.
 14. A systemaccording to claim 10, wherein the first wavelength of optical radiationis approximately 660 nm and the second wavelength of radiation isapproximately 880 nm.
 15. A system according to claim 10, wherein thedual wavelength optical sensor is disposed in the volume of bloodproximate an implantable medical device.
 16. A system according to claim15, wherein the implantable medical device comprises one of: animplantable heart monitor, a pacemaker, a cardioverter-defibrillator, amyostimulation device, a nerve stimulation device, a drug deliverydevice.
 17. A system according to claim 10, wherein said dual wavelengthoptical sensor mechanically couples to an implantable medical device.18. A system according to claim 10, wherein the oxygen saturationcomprises an arterial oxygen saturation metric.